Fig. 1. High-resolution XRD pattern of θ–2θ scan of GaN/AlN MQWs along the (0002) direction (black curve) with well-matched simulation (red dashed line) considering asymmetric thickness deviations.

Fig. 2. (a) STEM-HAADF image in cross-section view shows well-defined periodic structure of GaN/AlN MQWs grown on AlN buffer layer. High-resolution STEM-HAADF images taken from the (b) top and (c) bottom parts of a GaN QW stack indicate sharp interfaces. The bright (dark) contrast corresponds to GaN (AlN).

Fig. 3. STEM-CL local spectra of the GaN/AlN MQWs (filled red spectrum) and of the AlN layer (blue spectrum) taken at room temperature (RT) in the cross section of the sample. The main peak at 232 nm is assigned to the GaN/AlN MQWs with a weak and broad AlN luminescence from related oxygen DX centers found around 400 nm.

Fig. 4. (a) STEM-HAADF image of the first 13 GaN/AlN QWs with corresponding (b) panchromatic CL intensity image measured at RT.

Fig. 5. (a) Profiles of HAADF signal and (b) CL intensity (blue bar graph) with fit function (red curve), corresponding to the solution of the one-dimensional exciton diffusion model in an MQW, yielding a spatial resolution of σCL=1.8  nm and a diffusion length of λ=16  nm.